Magnetic resonance imaging for in-situ observation of the effect of depressurizing range and rate on methane hydrate dissociation

Abstract

Depressurization is considered to be the most promising method for exploitation of natural gas hydrate. To analyze the characteristics of hydrate dissociation during depressurization, methane hydrate (MH) dissociation was performed at different depressurizing ranges and rates, and the hydrate dissociation process was directly observed using magnetic resonance imaging (MRI). The experimental results indicate that with increased depressurizing rate from 0.01MPa/min to 0.1MPa/min, the average dissociation rate increases for a given depressurizing range. Meanwhile, with an increase in depressurizing range from 0.3MPa to 1.1MPa, the average dissociation rate increases for a given depressurizing rate. Moreover, the hydrate dissociation process can be divided into two main stages: hydrate saturation remains constant with little fluctuation for several minutes after back-pressure decreases, and then the hydrate dissociates continuously until dissociation completes. In addition, excessively high depressurizing range and rate result in hydrate reformation and ice generation, which slow the rate of hydrate dissociation. Furthermore, it was also determined that MH reformation and ice generation always occur at the higher depressurizing range and rate due to insufficient heat transfer.